13.2: Basins Formed by Crustal Extension and/or Thermal Sag

Rift Basins

Tectonics and crustal dynamics

Figure \(\PageIndex{1}\): Two map view sketches showing the progression from an initial three armed rift (triple junction) caused by thermal doming to the development of a new divergent boundary and failure of one of the rift arms (Page Quinton via Wikimedia Commons; CC BY-SA 4.0;  which is after DiPietro, 2018).

Rift basins form in extensional settings where the Earth’s crust is pulled apart and thinned.  This can happen either as passive rifting where the crust is pulled apart which results in the upwelling of the asthenosphere or as active rifting where heat builds up beneath the crust which causes uplift and then extension via gravitational sliding. 

Rift basins often begin as three-armed structures; clusters of these structures can exist in areas experiencing extension.  As time goes on, two of the arms are often successful in that extension continues and those arms link with other successful arms in nearby structures to form new divergent plate boundaries.  in these areas and those arms link up with successful arms in adjacent areas.  The third, “failed” arm or aulocogen is commonly filled with thick sedimentary deposits from the early phases of rifting.   

Rift basin development and sedimentation can be subdivided in several different phases throughout the life of the basin.  The earliest pre-rift phase takes place during the early phase of active or passive rifting where the area is thinned and uplifted ahead of major extension and faulting.  The syn-rift phase is characterized by active faulting, rapid subsidence and sedimentation, and commonly bimodal volcanism.  The post-rift phase is characterized by slower thermal subsidence; in failed rift arms this can result in the “steer’s head” model/morphology where more gradual and widespread sedimentation blankets a broader area.  The post-rift phase can also be accompanied by more widespread volcanism (inc. flood basalts) enabled by the upwelling asthenosphere and thinned crust. In areas where rifting was successful, rift basins eventually transition into a passive margin; these tectonically inactive continental margins are characterized by very slow thermal subsidence and widespread sedimentation across a large area that marks the transition between continental and oceanic crust.

Figure \(\PageIndex{2}\): Cross sectional sketches showing the architecture of rift basins and passive margins. Brown color indicates relatively sedimentation during active rifting and yellow color indicates slower sedimentation during post rift thermal subsidence (Page Quinton via Wikimedia Commons, CC BY-SA 4.0; which is after Bradley, 1982 and Transect passive margin.png)

Morphology

Rift basins are bounded by normal faults that are at a relatively high angle in the brittle rocks of the upper crust and become gently curved low-angle listric faults at depth.  This fault configuration causes tilted and rotated fault blocks along the margins of the basin.  Most rift basins are strongly asymmetric and form “half grabens” that are bounded by a steep border fault on one side that produces significant offset and rotation and a minor fault with minor displacement and relatively gentle rotation on the other side.  Arrays of bounding faults along basin margins can cause stepped “shoulders” along the edge of rift basins – especially on the most actively subsiding side.  In some places, rifting produces more symmetric faulting resulting in symmetric downdropped grabens and elevated horsts. 

Figure \(\PageIndex{3}\): Cross section showing grabens and half grabens developed in association with normal faults (Page Quinton via Wikimedia Commons; CC BY-SA 4.0;  which is after a diagram at GeologyIn.

Sedimentology

Broadly speaking, rift basins typically contain some combination of coarse-grained clastics near basin margins and early in the history of the basin), fine-grained clastics and/or evaporites in the central part of the basin, and volcanics facilitated by the thinned lithosphere and partial melting. The details of a particular basin can be profoundly influenced by climate, connectivity with adjacent rift basins, topographic elevation, proximity to the ocean, and success or failure of the rift arm.  Given their relatively small size and proximity to uplifted continental crust, rift basin sediments tend to be compositionally immature – especially in arid environments.    

During active rifting, high relief along basin margins enables the deposition of coarse-grained sediments (inc. diamictites, breccias, and conglomerates) in alluvial fans and fluvial systems.  Debris flow deposits are thickest and most common in proximal alluvial fan settings; they are commonly modified by, and transition downdip into, fluvial deposits.  Fluvial systems are composed of gravel and/or sand transported downdip from basin margins.  Transverse systems sourced by basin margin fans can link up to form through going axial systems in open basins with adequate rainfall.  Depending on sediment flux, climate, and connectivity to other bodies of water, basin centers can be dominated by fine-grained clastics (floodplain and/or lacustrine) or freshwater/marine evaporites.  Volcanism in rift basins is commonly mafic or bimodal (mafic and felsic with few/no intermediate rocks).  Mafic rocks are sourced by partial melting of the asthenosphere and can include flood basalts, sills, and dikes.  Felsic volcanism happens when rising magmas heat and partially melt continental crust resulting in rhyolites and/or tuffs.  Rift settings typically don’t enable the type of magma mixing, assimilation, or differentiation that produces intermediate rocks.

Coarse-grained deposition is more abundant and important during phases of active rifting earlier in the history of the basin.  During the post-rift phase, sedimentation becomes more widespread and fluvial, shallow lacustrine, and/or shallow marine dominates. 

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Passive Margins

Tectonics and crustal dynamics

Passive margins are the product of successful continental rifting and the development of a new divergent plate boundary.  As the continental margin moves away from the spreading center, it enters a prolonged period of tectonic quiescence and thermal subsidence.  Early in its history, a passive margin exhibits  rotated fault blocks (from preceding rifting), variable crustal thickness marking the transition from continental to oceanic crust, and variable topography with localized highs and depocenters.  As time goes on, variability diminishes as a vast prograding wedge of sediment buries remnant rift structures and forms extensive wedge of seaward dipping clinoforms. 

Morphology

Passive margins record the transition from rifting to tectonic quiescence and from continental to oceanic crust.  Once remnant fault blocks and rift basin sediments are buried, more mature passive margins develop the traditional continental margin morphology composed of a gently sloping continental shelf, the steeply dipping continental slope, and the modestly dipping continental rise.  Over tens to hndreds of millions of years, this wedge of sediment deposited in these environments thickens and progrades seaward forming a series of seaward-dipping clinoforms. 

Sedimentology

Given their geographic extent, longevity, and large potential accommodation, passive margin sediments are laterally extensive and thick.  Stratigraphic architecture is variable given the potential influence of climate, paleogeomorphic variability, and changes in sea level.   

Clastic-dominated coastlines contain the spectrum of environments described in Sections 10.3 and 10.4, including deltas, shorefaces, as well as the spectrum of shelf, slope, and rise deposits.  In warm areas with reduced clastic input, carbonate platforms can develop; they can include reefs, shoals, and the spectrum of deposits associated with carbonate ramps. 

The transition from rift to passive margin is accompanied by a progressive increase in compositional maturity as sediment is subjected to recycling, reworking and prolonged transport.  Although broadly progradational, modest subsidence rates  and proximity to the open ocean means that passive margin successions can be greatly influenced by eustasy can contain numerous transgressive-regressive cycles and a high fidelity sequence stratigraphic record.

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Back Arc Basins

Figure \(\PageIndex{4}\): Schematic cross section of a back arc basin ((Page Quinton via Wikimedia Commons; CC BY-SA 4.0 which is after Artemieva, 2023).

A back-arc basin forms in an extensional area formed behind a volcanic arc (on the overriding plate) in a convergent plate boundary.  It can form in either continental or oceanic crust.  The area behind the volcanic arc (on the opposite side of the arc from the trench) can experience extension because of slab rollback and/or a variety of other processes associated with the subducting slab and/or associated processes in the mantle.  In some cases, this extension can continue and lead to the development of a new spreading center. 

Back arc basins are elongate features that parallel the volcanic arc and have an overall geometry comparable to the early stages of a rift basin.  The sedimentology is also broadly comparable to a rift basin, however volcanic and marine rocks are more common and abundant and the source area is more intermediate in composition.

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Intracratonic Basins

Figure \(\PageIndex{5}\): Schematic cross section of an intracratonic basin (Page Quinton via Wikimedia Commons; CC BY-SA 4.0 which is after Leighton and Kolata, 1990).

Tectonic setting and crustal dynamics

Intracratonic basins form in the interior of continental cratons and are far removed from active plate boundaries.  The exact processes responsible for their formation are still debated, but they are most often attributed to some combination of thermal subsidence in association with older rifts, general lithospheric cooling, and/or mantle driven topography.  They tend to be long-lived features with relatively modest subsidence rates.    

Morphology

In map view, intracrstonic basins are circular to elliptical features with broad, subtle boundaries formed where sediments onlap the adjacent crust (as opposed to the sharp, fault-related boundaries associated with other structurally driven basins).  In cross-section view they have a symmetric, bowl-like shape wither laterally continuous stratigraphy and little/no structural complications. 

Intracratonic basins are typically broad, gently subsiding depressions with simple geometries. In plan view, they are commonly circular to elliptical, reflecting radially symmetric subsidence rather than linear, tectonically controlled deformation. Basin margins are subtle and gradational, lacking the sharp structural boundaries seen in rift or foreland settings.

Sedimentology

Their modest subsidence rate and structural simplicity mean that the fill of intracratonic basins is typically composed of extensive, laterally continuous deposits organized into a symmetric, bowl-shaped configuration.  If present, coarse-grained clastic sediment is relatively mature.  Carbonates, evaporites, and fine-grained clastics are abundant.  Intracratonic basins is dominated by regionally extensive, laterally continuous facies deposited under relatively stable conditions. Because subsidence is slow and long-lived, sedimentation is commonly controlled by eustatic sea-level fluctuations, climate, and sediment supply rather than rapid tectonic events.  Given their modest subsidence rates, intracratonic basins are particularly sensitive to eustatic and climatic influences on stratal architecture. 

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Readings and Resources

• Stretching the lithosphere; Rift basins

• Nascent conjugate, passive margins

• Artemieva, I.M., 2023, Back-arc basins: A global view from geophysical synthesis and analysis: Earth-Science Reviews, v. 236, p. 104242.

• Bradley, D.C., 1982, Subsidence in Late Paleozoic basins in the northern Appalachians: Tectonics, v. 1, p. 107–123, doi:10.1029/TC001i001p00107.

• Buschbach, T.C., and Kolata, D.R., 1990, Regional Setting of Illinois Basin, in Leighton, M.W., Kolata, D.R., Oltz, D.F., Eidel, J.J., and Coury, A.B. eds., Interior Cratonic Basins, American Association of Petroleum Geologists, v. 51, p. 29–55.

• Buschbach, T.C., and Kolata, D.R., 1990, Regional Setting of Illinois Basin, in Leighton, M.W., Kolata, D.R., Oltz, D.F., Eidel, J.J., and Coury, A.B. eds., Interior Cratonic Basins, American Association of Petroleum Geologists, v. 51, p. 29-55. 

• Buschbach, C.R., Jr., 1991, The Michigan basin, in Gluskoter, H.J., Rice, D.D., and Taylor, R.B. eds., Economic Geology, U.S., Geological Society of America, v. P-2, p. 287–300.

• DiPietro, J.A., 2018, Chapter 5 - Forcing Agent: The Tectonic System, in DiPietro, J.A. ed., Geology and Landscape Evolution (Second Edition), Elsevier, p. 59–77.

• Ettensohn, F.R., Pashin, J.C., and Gilliam, W., 2019, Chapter 4 - The Appalachian and Black Warrior Basins: Foreland Basins in the Eastern United States, in Miall, A.D. ed., The Sedimentary Basins of the United States and Canada (Second Edition), Elsevier, p. 129–237.

• Horton, B.K., Capaldi, T.N., Mackaman-Lofland, C., Perez, N.D., Bush, M.A., Fuentes, F., and Constenius, K.N., 2022, Broken foreland basins and the influence of subduction dynamics, tectonic inheritance, and mechanical triggers: Earth-Science Reviews, v. 234, p. 104193

• Leighton, M.W., and Kolata, D.R., 1990, Selected Interior Cratonic Basins and Their Place in the Scheme of Global Tectonics: A Synthesis, in Leighton, M.W., Kolata, D.R., Oltz, D.F., Eidel, J.J., and Coury, A.B. eds., Interior Cratonic Basins, American Association of Petroleum Geologists, v. 51, p.727-797.

• Miall, A.D., and Catuneanu, O., 2019, Chapter 9 - The Western Interior Basin, in Miall, A.D. ed., The Sedimentary Basins of the United States and Canada (Second Edition), Elsevier, p. 401-443.

• Olsen, P.E., 1990, Tectonic, Climatic, and Biotic Modulation of Lacustrine Ecosystems—Examples from Newark Supergroup of Eastern North America, in Katz, B.J. ed., Lacustrine Basin Exploration: Case Studies and Modern Analogs, American Association of Petroleum Geologists, v. 50, p. 209-224.

• Selley, R.C., and Sonnenberg, S.A., 2015, Chapter 8 - Sedimentary Basins and Petroleum Systems, in Selley, R.C. and Sonnenberg, S.A. eds., Elements of Petroleum Geology (Third Edition), Boston, Academic Press, p. 377-426.

• Sheridan, R.E., 1989, The Atlantic passive margin, in Bally, A.W. and Palmer, A.R. eds., The Geology of North America—An Overview, Geological Society of America, v. A, p. 81-96.

• Stern, R.J., and Dickinson, W.R., 2010, The Gulf of Mexico is a Jurassic backarc basin: Geosphere, v. 6, p. 739–754.